The present invention relates generally to muscle exercise apparatus and more specifically to exercise apparatus capable of providing both positive and negative exercise over a range of motion.
A muscle produces force when it contracts. One form of exercise is called isometric exercise; the muscle length remains constant as the muscle contracts against force applied by an opposing muscle or against an immovable object.
Other forms of exercise involve shortening or lengthening a muscle through a range of movement of a limb about a joint. Movement in the direction of the muscle contracting force against an external resistance shortens the muscle and is called concentric contraction. Movement caused by a greater external force in a direction opposite to the muscle contracting force lengthens the muscle and is called eccentric contraction. Concentric contraction is known as positive exercise; eccentric contraction is called negative exercise.
Since isometric exercise pits a muscle against another muscle or against an immovable object, no special equipment is needed. Most exercise equipment, therefore, is not of this type, although many dynamic machines may also be used in the static mode to provide isometric exercise.
The most common type of exercise apparatus uses weights or their equivalent to provide isotonic exercise, in which a constant external resistance force is applied during a dynamic contraction, so that the speed of movement varies in response to the varying muscle force output at each point of a range of motion.
The geometric relationship between muscle anchoring points and joint locations, however, normally results in a maximum output force at some intermediate point in the range of motion of a given limb as it is moved by muscles about its joint. Thus, when using a pure isotonic exercise apparatus, such as a barbell or a stack of rail-guided weights lifted by a cable, the weight selected for exercising a given muscle or muscle group over a range of motion of the corresponding limb is limited by the force that can be exerted at the weakest point in the range of motion. Consequently the muscle or muscle group exerts less than its maximum potential force at all points of the range of motion except the weakest point.
Simple weight lifting devices also have the potential to cause muscle injury when the full mass of the weight is being accelerated at the start of the lift. If the weight is supported by a spring, then the resistive force of the mass/spring systems increases gradually until the compressed spring reaches its neutral position. U.S. Pat. No. 5,117,170 of Keane et al. discloses a control circuit for an electric motor to produce a counterforce upon rotation of the motor shaft from a zero position that simulates a weight stack supported by a spring.
Another type of exercise device uses springs instead of weights to provide a resisting force. A spring that has been displaced from its neutral position exerts a restoring force that directionally opposes and linearly varies with the displacement. Exercise machines based on springs for the provision of force are thus capable of providing both positive (concentric contraction) and negative (eccentric contraction) exercise over a range of motion. However, the monotonically rising straight line force curve of a conventional linear spring also does not match the force/displacement curve of a muscle-actuated limb/joint combination. This has tended to limit the utility of spring-based exercise apparatus.
A further type of exercise device known as an isokinetic machine was developed. In isokinetic exercise, the speed of the exercise motion is held constant during contraction. Such devices generally do not provide negative resistance, even though negative resistance is very desirable in many exercise regimes.
Examples of exercise machines are set forth in U.S. Pat. Nos. 3,465,592 to Perrine, 5,011,142 to Eckler, 4,261,562 to Flavell, and 5,180,351 to Ehrenfried, the contents of which are incorporated herein by reference.
Some experts believe that a muscle must be pushed to its maximum strength limit to derive maximum muscle hypertrophy. This approach calls for repetitive cycles of concentric contraction and eccentric contraction against a level of resistance until reaching a point of momentary muscle failure. The user then reduces the level of resistance and resumes the workout until a second momentary muscle failure is reached. The steps of resistance reduction leading to momentary muscle failure are repeated until the muscle reaches its absolute fatigue point, at which the muscle is incapable of working against resistances as low as 10% of the initial resistance of the workout.
The variables to consider in designing a workout program also include the time interval for each portion of an exercise cycle. Some experts believe that two seconds of positive (concentric) contraction followed by four seconds of negative (eccentric) contraction is optimal. Others maintain that a briefer, higher power concentric contraction of very short duration, followed by isometrically restraining an imposed load until muscle failure forces the lowering of the load, is the most effective.
There remains a need, therefore, for a versatile exercise machine that incorporates many of the advantages present in various prior art machines without their disadvantages. Ideally, such a machine should permit the user a broad range of exercise regimes.